Dielectric control of reverse intersystem crossing in thermally-activated delayed fluorescence emitters

TL;DR

This study demonstrates how the dielectric environment influences reverse intersystem crossing in TADF emitters, significantly enhancing rISC rates by stabilizing charge transfer states and reducing activation energy.

Contribution

It reveals the critical role of dielectric surroundings in modulating rISC processes in TADF materials, supported by experimental and quantum-chemical analyses.

Findings

Dielectric environment reduces singlet-triplet energy gap in dipolar TADF emitter.

Environmental reorganization triggers formation of CT product state.

Dielectric environment can increase rISC rate by up to three orders of magnitude.

Abstract

Thermally-activated delayed fluorescence (TADF) enables organic semiconductors with charge transfer (CT)-type excitons to convert dark triplet states into bright singlets via a reverse intersystem crossing (rISC) process. Here, we consider the role of the dielectric environment in a range of TADF materials with varying changes in dipole moment upon optical excitation. In a dipolar reference emitter, TXO-TPA, environmental reorganisation after excitation in both solution and doped films triggers the formation of the full CT product state. This lowers the singlet excitation energy by 0.3 eV and minimises the singlet-triplet energy gap ({\Delta}EST). Using impulsive Raman measurements, we observe the emergence of two (reactant-inactive) modes at 412 and 813 cm-1 as a vibrational fingerprint of the CT product. In contrast, the dielectric environment plays a smaller role in the electronic excitations of a less dipolar material, 4CzIPN. Quantum-chemical calculations corroborate the appearance of these new product modes in TXO-TPA and show that the dynamic environment fluctuations are large compared to {\Delta}EST. The analysis of the energy-time trajectories and the corresponding free energy functions reveals that the dielectric environment significantly reduces the activation energy for rISC, thus increasing the rISC rate by up to three orders of magnitude when compared to a vacuum environment.